[feat](trx-backend-soapysdr): add hardware AGC toggle and SDR settings UI row

Add hardware AGC on/off control for SoapySDR backend, wired through the
full stack from RigCommand to the web UI:

- RigCommand::SetSdrAgc(bool) + ClientCommand::SetSdrAgc in protocol
- set_sdr_agc() on RigCat trait (not-supported default)
- SoapySdrRig: agc_enabled field, set_sdr_agc() via pipeline agc_cmd,
  sdr_agc_enabled in filter_state(); removes the "not yet implemented"
  warning — gain_mode="auto" now properly enables hardware AGC via
  SoapySDR set_gain_mode()
- IqSource::set_gain_mode() trait method; RealIqSource implements it
- SdrPipeline: agc_cmd channel, read loop applies it each iteration
- POST /set_sdr_agc endpoint in trx-frontend-http
- New "SDR settings" full-row in index.html with Hardware AGC checkbox
  and RF Gain (moved out of WFM controls); row hidden when
  show_sdr_gain_control is false
- app.js: AGC checkbox handler, disables RF gain input while AGC is on,
  syncs checkbox state from filter.sdr_agc_enabled

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-core/src/rig/command.rs

@@ -41,6 +41,7 @@ pub enum RigCommand {
     SetBandwidth(u32),
     SetFirTaps(u32),
     SetSdrGain(f64),
+    SetSdrAgc(bool),
     SetSdrSquelch { enabled: bool, threshold_db: f64 },
     SetWfmDeemphasis(u32),
     SetWfmStereo(bool),

src/trx-core/src/rig/controller/handlers.rs

@@ -523,6 +523,7 @@ pub fn command_from_rig_command(cmd: RigCommand) -> Box<dyn RigCommandHandler> {
         | RigCommand::SetBandwidth(_)
         | RigCommand::SetFirTaps(_)
         | RigCommand::SetSdrGain(_)
+        | RigCommand::SetSdrAgc(_)
         | RigCommand::SetSdrSquelch { .. }
         | RigCommand::SetWfmDeemphasis(_)
         | RigCommand::SetWfmStereo(_)

src/trx-core/src/rig/mod.rs

@@ -190,6 +190,16 @@ pub trait RigCat: Rig + Send {
         )))
     }
 
+    fn set_sdr_agc<'a>(
+        &'a mut self,
+        _enabled: bool,
+    ) -> Pin<Box<dyn Future<Output = DynResult<()>> + Send + 'a>> {
+        Box::pin(std::future::ready(Err(
+            Box::new(response::RigError::not_supported("set_sdr_agc"))
+                as Box<dyn std::error::Error + Send + Sync>,
+        )))
+    }
+
     fn set_sdr_squelch<'a>(
         &'a mut self,
         _enabled: bool,

src/trx-core/src/rig/state.rs

@@ -313,6 +313,8 @@ pub struct RigFilterState {
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub sdr_gain_db: Option<f64>,
     #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub sdr_agc_enabled: Option<bool>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub sdr_squelch_enabled: Option<bool>,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub sdr_squelch_threshold_db: Option<f64>,